1. Field of the Invention
This invention relates to optical attenuators which may be either adjustable or controlled by a modulating signal in order to obtain accurate attenuation of a beam of light rays and is more particularly directed to an optical attenuator which provides controlled attenuation.
2. Description of Prior Art
Fixed or variable optical attenuators have already been proposed and are designed on the basis of many different techniques such as absorption, diffusion, polarization and so on, depending on their intended use. For example, polychromatic beams are attenuated in the field of image projection whereas monochromatic radiations are transmitted in the field of optical fiber transmission.
One example of a wide-band attenuator which can be employed as a light modulator for the projection of images, for example, is described by M. Hareng in "Revue Technique Thomson-CSF 3 No. 3, 473 (1971)" and consists of a liquid-crystal device having electrically controlled attenuation.
A desirable objective in the field of optical fiber transmission consists in subjecting a light signal delivered by an input fiber to attenuation by a perfectly known and variable quantity, the signal being collected by an output fiber after attenuation. Attenuation values as high as 50 db may thus be obtained. At high levels of this order, attenuators must achieve a very high quality standard in order to obtain the requisite degree of accuracy. Attenuators of this type do in fact exist: for example, in "Nec Research and Development, 50, 17 (1978)", K. Doi et Al describe attenuators obtained from plates providing variable optical density and mechanical insertion in either discrete or continuous values.
Another attenuation technique is described in French Pat. No. 77 39 196 filed on Dec. 26th, 1977 and relating to a "Variable optical attenuator for optical fiber transmission". In this case, attenuation is obtained from a plate cut from material having electrical birefringence controlled by the voltage applied between two electrodes.
In these examples, displayed values rely on calibration prior to use and do not account for possible variations in the attenuation value during operation. These variations may arise in the first case from mechanical displacements and in the second case from variations in electrical control or in real attenuation of the plates as a function of temperature, for example.
In order to limit the potential danger of variation of the attenuator, it is necessary to improve the quality of materials, to achieve higher accuracy in assembly and so on, with the result that the cost price of the attenuator is also increased.
The aim of the invention is to overcome this disadvantage by proposing an attenuator having a value of attenuation which is not dependent on preliminary calibration.